1. Field of the Invention
The present invention relates to a signal processing circuit for an encoder of a type having two detectors arranged on both sides of a rotating shaft in order to eliminate an eccentricity error.
2. Description of the Related Art
One of the detection errors that can occur in an encoder is an eccentricity error, i.e., the error caused by the eccentricity of its rotating part. This eccentricity error can be canceled by arranging two detectors on both sides of the rotating shaft and by taking an average between the angular values θ1 and θ2 calculated from their respective output signals, as described, for example, in the prior art section of JP 3195117B.
As the average θ=(θ1+θ2)/2 is the average of angular values, it is not the mere average of the values, but the value of θ is chosen so that, after the averaging, the angle θ is located between θ1 and θ2, as shown in FIGS. 1 and 2.
However, in a certain type of encoder, such as a magnetic encoder, in which the detectors are mounted to the stator or rotor after the encoder is assembled, the difference between θ1 and θ2 calculated from the output signals of the respective detectors may become equal to 180°, depending on the mounting condition of the detectors; in that case, two solutions θ′ and θ″ can occur, resulting in a 180° displacement, as shown in FIG. 3.
Therefore, to avoid the above situation, in the prior art the detectors have had to be mounted by exercising care so that the difference between θ1 and θ2 will not become equal to 180° while considering the positional relationship between the detectors, and hence the problem that it takes time to mount the detectors appears.